Net Income Per Share (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net Income Per Share [Abstract]
Net income, as reported, for basic net income per share	$ 15.6	$ 20.1	$ 44.8	$ 39.7
Plus: interest expense on convertible debt, net of tax	0.8	1.1	1.9	2.1
Net income for diluted net income per share	$ 16.4	$ 21.2	$ 46.7	$ 41.8
Weighted average common shares outstanding (in shares)	34.0	33.6	33.9	33.5
Assumed stock options exercised and awards vested, based on the treasury stock method (in shares)	0.5	0.5	0.5	0.5
Assumed conversion of convertible debt, based on the if-converted method (in shares)	2.1	2.9	2.5	2.9
Weighted average shares assuming dilution (in shares)	36.6	37.0	36.9	36.9
Antidilutive options excluded from computation of diluted net income per share (in shares)	0.6	1.0	1.7	1.2